Higher soil acidification risk in southeastern Tibetan Plateau

Stable soil pH is a key property in maintaining an ecosystem's structure, function, and sustainability. Increasing atmospheric deposition and grassland use on the Tibetan Plateau (TP) may increase the soil acidification risk, but we lack such information to date. Here, we evaluated the soil acidification risk in the TP, by comparing it with that in the Mongolia Plateau (MP) and applying the acid-base balance principles on atmospheric inputs, soils, and plants from 1980 to 2019. Cumulative acid input was lower in the TP than in the MP. Sulfur contributed more to acidity than nitrogen and atmospheric deposition contributed more to acidity than grassland use. Acid input was mainly influenced by local industry, animal husbandry and transportation in the MP, while in the TP it was also affected by the long-distance transportation of pollutants from South Asia and southern China. Overall, the TP was less acid-sensitive than the MP because of higher inorganic carbon content. However, soils in the southeastern TP, covering 21% of the total area, were acid-sensitive due to low levels of soil exchangeable base cation (EBCs) and lack of calcium carbonate. Coincidentally, the southeastern region has the highest concentration of acid input in the TP due to more rapid development and stronger influence of adjacent high acid deposition regions than others. Therefore, the acidification risk to the southeastern region is much higher than to other regions of the TP and the MP; in this region, the EBCs are likely to be depleted approximately 95 years earlier than in the MP. The findings of this study provide insights into the response of the TP to global change. For the ecosystem sustainability of southeastern TP, control of atmospheric acid deposition, especially sulfur deposition, in both local and adjacent regions and nations is required.


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Soil pH significantly influences soil biogeochemical cycles, biodiversity, productivity, 26 and many other factors in terrestrial ecosystems (Chytrý et al., 2007;Kirk et al., 2009). In the 27 past decades, the release of SO2 and NOx from fossil fuel combustion and high-energy 28 combustion has led to a two-to seven-fold increase in atmospheric acid deposition relative to 29 the time before the industrial revolution (Barak et al., 1997;Lu et al., 2014). Moreover, the 30 emission of NH3, which has strong acidification potential, has also increased rapidly with the 31 development of agriculture and transportation (Du et al., 2015). Meanwhile, increased use of 32 grasslands or forests for, such as, grazing, mowing, and harvesting, has removed base cations 33 from the soil and further exacerbated soil acidification (Bolan et al., 1991;Fujii et al., 2012). 34 The exposure of terrestrial ecosystems to long-term high levels of anthropogenic acid input 35 may lead to soil acidification and significantly change ecosystem structure and function (Chen 36 et al., 2013a). Understanding the risk of soil acidification is important to maintain the stable 37 and sustainable development of regional ecosystems. 38 Changes in soil pH are generally assumed to reflect the level of soil acidification (Ji et   and alterations in rainfall (Zheng et al., 2014). Unfortunately, the response of grassland soils 64 in the TP to acid input, particularly to the increasing atmospheric acid deposition and 65 grassland utilization, has not received much attention. This is because previous studies argued 66 that the soils in the TP were high in inorganic carbon (Mi et al., 2008;Yang et al., 2012), 67 indicating a low sensitivity to acidity. However, the inorganic carbon content in the TP soils is 68 spatially heterogeneous; soils with high inorganic carbon content mainly occur in the TP's 69 northwestern regions, with much lower levels in the southeastern regions. Coincidentally, the 70 southeastern region is the major development area of the TP owing to its lower elevation and 71 higher temperatures than other TP areas (Figs. S1). Furthermore, influenced by East Asian and 72 South Asian monsoon (Fig. 2), it has relatively high levels of atmospheric acid deposition Third Pole region is a prerequisite for maintaining its sustainable development.

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For effective assessment of the soil acidification risk in the TP, we selected the 78 Mongolian Plateau (MP) for comparative study (Fig. 2). The Mongolian Plateau is the most 79 representative temperate grassland in the northern hemisphere. Compared with the TP, the MP 80 has more developed industries and agriculture, and a higher ratio of carbonate area to total 81 area. Different levels of human activity and soil acidity buffer capacity may lead to 82 differences in their acidification risks. Meanwhile, due to different regional emission 83 reduction strategies, the contribution of nitrogen and sulfur deposition to their acidity differs, 84 and therefore quantification of contributions from different sources can provide guidance for 85 future policy-making. Thus, the main objectives of the study were to: (1) quantify the levels 86 of acid input and the contribution ratio of different factors between the two comparative 87 plateaus; (2) ascertain the sensitivity to acid input of grassland soils between the two plateaus;

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(3) compare the durations of depletion of different acid buffering pools in the TP with those in 89 the MP. In particular, the assessment of soil acidification and its influencing factors may aid in 90 formulating better strategies to manage existing policies and the regulatory framework to 91 improve the region's socioeconomic development while reducing the risk of acidification.
where NH4,L + and NO3,L − are the leaching amounts of NO3 − and NH4 + , respectively. Since the 123 mobility of NH4 + in the soil is poor, and most of it is absorbed by plants or nitrated, we 124 assume that no significant amounts of NH4 + is leached from the soil (Duan et al., 2004). The where ND, NU, and Nde are the deposition of total nitrogen, plant absorption, and 128 denitrification, respectively. NL is the leaching amount of total nitrogen (since the leaching of 129 NH4 + is assumed to be 0, the leaching amount of total nitrogen is equal to that of NO3 − ).

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Through our field surveys, we found that legumes were not the dominant species in the study 131 area, so we did not consider the nitrogen fixation of legumes.

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where the sum of NHx and NOy is the total nitrogen deposition; Nde is the N that undergoes where BCupt (kmolc ha -1 yr -1 ) is the net loss of base cations (BC) from grazing and mowing 151 grasslands. We did not calculate the N-uptake-induced proton caused by nitrogen absorption 152 in this part because it has been calculated in atmospheric deposition.
where BCS and BCL (mmolc g -1 ) are the element contents of BC in the stems and leaves,   to the total grassland area was also higher in the MP than the TP (56% vs. 25%). In both areas, 208 S and N deposition contributed approximately 60% and 40% to acid inputs, respectively.

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Furthermore, base cations deposition and part of the N cycling process offset 55% and 18% of 210 the potential acidity, respectively (Table 1).

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The inputs of H + from atmospheric deposition varied over time, and showed different grassland utilization to acid input was much smaller than that of atmospheric deposition.

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However, the contribution of grassland utilization to the proportion of H + input area to the 232 total grassland area was much larger than that of atmospheric deposition ( Fig. 3B and C).  (Fig. S6). Conversely, EBCs in non-carbonate-containing soils were significantly lower 244 in the TP than in the MP, but the ratio of non-carbonate-containing soil area to total soil area 245 was higher in the TP than in the MP (36% vs. 3%, respectively) (Fig. 5A). 246 We estimated that, in the past 40 years, the total acid inputs have consumed only 0.4% of 247 the soil calcium carbonate in the TP and 5% in the MP (Fig. S7A). However, over that period,

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EBCs decreased by 21% in the TP and 12% in the MP; approximately 1313 km 2 of soil was 249 depleted of EBCs in the TP, which resulted in the release of Al 3+ and Mn 2+ (Fig. 5A). In both the TP and the MP, calcium carbonate in carbonate-containing soils can last for 253 several thousand years (Fig. S7B). Carbonate-containing soils accounted for 63% and 94% of 254 the soils exposed to the area of acid input in the TP and the MP, respectively. However, in the 255 TP, high acidity inputs were mainly concentrated in non-carbonate-containing soils, which led 256 to higher acidification risk than in the MP. The EBCs may be depleted from these soils in 257 approximately 312 years (Fig. 5B). In addition, the period until the depletion of these buffer 258 systems in different grassland types shown in Table 2 showed that, of all the grasslands, the 259 steppe had the highest acidification risk.      Both flux and the area receiving acid input as a proportion of the total area of cumulative acid 526 input were lower in the TP than in the MP. Atmospheric deposition contributed more to 527 acidity but less to the area receiving acid input than grassland use types.